Redox Mediators in Next Generation Metal-Oxygen Batteries: A Systematic Study on Homogeneous Catalysts for Li-, Na-, and Zn-O2 Cells

Abstract

Non-aqueous metal-oxygen batteries are very attractive for future energy storage due to their high theoretical energy densities. However, there are major drawbacks that hinder the commercial realization of these batteries. Amongst other challenges, high charging potentials for Li-O2 cells, the ambiguity how to steer the cell reaction between one- and two-electron transfer for Na-O2 cells, and the poor cycling stability for non-aqueous Zn-O2 cells prevail even in academic research. In order to mitigate these drawbacks, the use of redox mediators (RMs), which are dissolved in the liquid electrolyte, emerges as one favorable strategy. RMs have been recently proposed as highly promising, homogeneous catalysts for the decomposition of discharge products enabling a significant reduction of the charging overpotentials [1]. To reduce the charging overpotential is mandatory to propel Li-O2 systems and is thus attracting a great deal of interest. However, the use of RMs in Na-based systems, for which the charging overpotentials are significantly lower, has been less studied, and, to the best of our knowledge, no study has been reported for RMs in Zn-O2 cells up to now.In this work, we present a systematic analysis of RMs in Li-, Na- and Zn-O2 cells. We show that the appropriate choice of electrolyte and type of RM can enable the oxidation of discharge product deposited at the cathode side even without direct electric contact. In detail, the incorporation of only 10 mM TEMPO (2,2,6,6,-Tetramethyl-1-Piperidinyloxy) into Li-O2 cells provides a distinct reduction of the charging potentials by 500 mV. Moreover, TEMPO enables a significant enhancement of the cycling stability, doubling the cycle numbers compared to a Li-O2 cell without RM [2]. Furthermore, we investigate the impact of the RM NaI (sodium iodide) in Na-O2 cells, and will discuss the impact of its concentration (0.05 – 2.00 M) on cell performance by means of impedance spectroscopy and galvanostatic cycling. Interestingly, NaI can be applied as conducting salt and RM at the same time in these cells. In addition, we show that adding 10 mM of the RM DMFc (Decamethylferrocene) is beneficial for the cycling of Zn-O2 cells with the ionic liquid 1-Ethyl-3-methylimidazolium dicyanamide as electrolyte.We include detailed investigations of the relevant chemical and physical properties of the respective RM, such as the electrochemical stability (analyzed by using cyclic voltammetry). Moreover, we study the discharge/charge reactions in all aforementioned metal-oxygen batteries by means of X-ray diffraction analysis, Raman spectroscopy, and SEM imaging to discuss possible side reactions and the stability of the RM, as well as the electrolyte. All in all, the results obtained will contribute to the optimization of other redox active compounds and might support further development of metal-oxygen batteries as next generation energy storage.